SnAg alloy solder is used as a PbSn solder substitute material in bonding materials for use in semiconductor substrates (wafers) or printed boards. An SnAg alloy plating solution for forming a SnAg alloy by a plating method is formed of an aqueous solution obtained by dissolving a tin compound and a silver compound. When tin or silver dissolved in this SnAg alloy plating solution is precipitated into the plating solution as metal or an insoluble salt, the alloy composition of the SnAg alloy plating film formed on an object to be plated will not become the composition as planned, and the precipitate causes defective gloss or uneven gloss of the plating film. In the SnAg alloy plating solution, since silver is a more noble metal than tin, an oxidation reaction of Sn2+ ions and a reduction reaction of Ag+ ions occur in the plating solution, and silver tends to be easily precipitated out. Therefore, in order to stably dissolve the silver compound in the SnAg alloy plating solution, a sulfur-containing compound such as a heterocyclic compound having a mercapto group or a sulfide compound is added as a complexing agent to the solution, and thereby a complex of silver is formed.
Patent Document 1 discloses a plating solution including a nitrogen-containing heterocyclic compound substituted with a mercapto group. In this Patent Document 1, 1-(2-dimethylaminoethyl)-5-mercapto-1,2,3,4-tetrazole is listed as an example of the nitrogen-containing heterocyclic compound.
Patent Document 2 discloses a plating solution including a mercaptotetrazole derivative and dithiaalkyl diol.
Patent Document 3 discloses a plating solution including a particular sulfide-based compound such as 2,2′-dipyridyl sulfide or 2,2′-dipiperazinyl disulfide, which has one or more basic nitrogen atoms in the molecule, and a plating solution including a particular thio crown ether compound such as 1-aza-7-oxa-4,10-dithiacyclododecane.
Patent Document 4 discloses a plating solution including a sulfur-containing compound that has a particular monocyclic heterocyclic group or fused heterocyclic group, both of which contains 1 to 5 units of at least one atom selected from nitrogen, sulfur, and oxygen, and has a sulfide group or a mercapto group adjacent to the heterocyclic group.